[0001] The present invention relates to a device for measuring fluid flow and to a circuit
for operating such a device.
[0002] Our published European Patent Application number 0239703 describes a flow sensing
device in the form of a substrate of semiconductor material fabricated with a micro-engineered
cantilever beam. It includes a means of sensing a characteristic of the beam which
is indicative of fluid flow relative to the beam. This device has a limited range
of flow rates capable of being measured, and in order to cover a wide range of flow
rates, several such devices with different dimensions must be incorporated on the
same silicon chip. Furthermore, to enable measurement of low flow rates, the cantilever
beam must be thin and this can lead to problems with stress causing the beam to bend.
Another disadvantage of this invention is that if the preferred capacitance variation
measurement is used to determine the response of the cantilever beam to fluid flow,
it is necessary that the fluid density be assumed constant. This enables the deflection
of the cantilever beam to be considered dependent on the fluid velocity and independent
of fluid mass. In practice, the density of the fluid will not be constant and a means
of compensating for changes in fluid dielectric constant would be required.
[0003] It is thus an object of the present invention to provide a fluid flow sensing device
which is suitable for production by means of the integrated circuit technology commonly
known as micromachining, with advantages well known to that technology, and which
at least alleviates some of the disadvantages of the prior art. A micromachined device
is one which is constructed from some other part of the same device, and forms an
integral part of it.
[0004] According to the present invention there is provided a flow sensing device comprising
a substrate of semiconductor material and a micromachined beam, the device having
first and second major surfaces, the first surface further defining a pivot about
which the beam is able to twist under the action of fluid flow. Thus, by constructing
part of the beam from the device material, manufacturing the device is made easier
than with prior art devices.
[0005] Preferably the device is constructed so that there is a channel below one half of
the beam which extends throughout the substrate, and there is a cavity below the other
half of the beam, such that during operation fluid may flow through the channel thus
exerting a force on one half of the beam and fluid fills the cavity thus creating
a differential force across the beam. The device thus operates on a "null" principle
in that a force is applied to the beam in order to compensate for the differential
force caused by the effect of fluid flow upon the beam, this compensating force being
indicative of the fluid flow rate.
[0006] In a further embodiment of this invention, the device is fabricated in a fully planar
fashion and thus, unlike many prior art devices, negates the need for a second substrate
to be added to the original substrate sandwiching the sensing beam between the two
substrates.
[0007] In yet a further embodiment of this invention, there is provided a circuit suitable
for operating such a device.
[0008] In yet a still further embodiment of this invention, there is provided a method of
manufacturing such a flow sensing device.
[0009] In order that the invention may be clearly understood and readily carried into effect,
it will be described, by way of example only, with reference to the accompanying drawings,
of which:
Figures 1A, 1B, 1C and 2 are schematic representations of a micromachined flow sensor.
Figure 3 is a block diagram of a typical circuit to operate the device.
Figures 4A, 48, 4C and 4D schematically represent stages in a preferred method of
fabrication of the device.
[0010] Figure 1 shows a flow sensor in accordance with the present invention comprising
a beam 1 fabricated from a thin insulating film 2 typically 0.5 micrometres thick,
this film 2 is coated with a metal 3 which is typically 3 micrometres thick. This
metal 3 ensures that the beam 1 is both rigid and flat. The beam 1 is pivoted, in
this example, about its mid-point by supports 4 constructed from, and integral with,
the same insulating layer as the beam. The beam is able to pivot about the axis X-X,
and the pivot supports 4 are thin, typically 0.5 micrometres, so as not to affect
the sensitivity of the device at low flow rates. The beam is supported above a semiconductor
substrate 31.
[0011] Electrical contacts 11A, 11B are formed on the free ends of the beam, these act as
one electrode of a capacitor. The other capacitor electrodes 6 are fabricated directly
below these.
[0012] Below one half of the beam a channel 7 is formed which extends through the semiconductor
substrate 31 from the first major surface of the substrate 9 on which the beam 1 is
fabricated, to the opposite major surface 10 of the substrate. Thus the fluid whose
flow is being measured is able to pass right the way through the substrate 31. A cavity
8 is formed below the other half of the beam 1. This cavity extends only part way
through the substrate 31 and does not allow the passage of fluid through it. This
then allows the device to operate on a "null" principle. This means that when the
device is put in the path of a flowing fluid, because only one "through" channel exists,
there will be a differential force across the beam and only the side of the beam above
this "through" channel will experience this force.
[0013] In operation, a component of fluid flow normal to the major surfaces of the beam
1 in either direction will cause the beam 1 to twist about its pivot point, with the
half of the beam above the "through" channel moving in the direction of the component
of fluid flow acting upon it.
[0014] An electrostatic force is applied to the device contacts 11A, 11B in such a way as
to return the beam to its equilibrium position. This equilibrium position is defined
as being when the contact gap at zero fluid flow (Yo in Figure 2) is equal on both
sides of the beam, i.e. between 11A and 6 and 11B and 8.
[0015] The amount of electrostatic force required and the contacts to which it is applied,
is determined by comparing the capacitance between the contacts 11A, 6 on one end
of the beam to those on the other end of the beam 11B, 8. In this way changes in dielectric
constant of the fluid do not affect the measurement of fluid flow. The upper limit
of the range of fluid flow rates over which the device is capable of working is limited
only by the amount of voltage available to apply sufficient electrostatic force.
[0016] In this way the fluid velocity may be obtained by utilizing the equation of motion
of the overall system and standard feedback control theory. In practice, the deflection
of the beam is small and the restoring force of the hinges is small compared to the
force due to the fluid flow.
[0017] Referring now also to Figures 2 and 3,

where:-
V = Resulting Electrostatic force bringing the beam back to equilibrium.
= Density of fluid (Kg/m³)
v = velocity of fluid (m/s)
b = width of beam (m)
r = length of beam to hinge (on one side) (m)
yo = contact gap at zero fluid flow (m)
= permittivity of free space
A = Area of overlap of top and bottom contacts.
[0018] A functional block diagram of one suitable circuit for operating such a device is
shown in Fig. 3.
[0019] A sine wave voltage V
o is applied across the contacts 11A and 11b. Amplifiers 12A and 12B convert the capacitances
between these contacts into a voltage given by:-

where C1 and C2 are the capacitances between the electrical contacts of the beam
11A, 11B and the electrodes 6 respectively.
[0020] C
F is the feedback capacitance around the amplifiers 12A and 12B.
[0021] After amplification by amplifiers 13A and 13B and rectification by 14A and 14B the
voltages are compared in a difference amplifier 15. The gain of 13B is adjusted so
that with zero fluid velocity the output from the difference amplifier is zero.
[0022] The output from the difference amplifier is of positive or negative sign depending
upon whether capacitance C1 or C2 is the greater. The switch 17 is designed to apply
the amplified error voltage from 16 to the contacts with the smallest capacitance,
the resulting electrostatic force bringing the beam back to its equilibrium position.
[0023] The square root function performed by 18A and 18B is an important addition to this
circuit as it ensures the equation of motion for the overall system (including the
flow sensor) is in the form of a linear differential equation, and hence is readily
analysed using standard feedback control theory.
[0024] Figures 4A to 4D illustrate a preferred method of fabrication of the device, the
method is described as follows:
a) A monocrystalline silicon wafer 20 typically 350 micrometers thick is prepared
with the major surfaces lying in the 100 plane.
b) A layer 21 of an electrically insulating material e.g. silicon dioxide, silicon
oxynitride or silicon nitride, typically 0.5 micrometres thick is deposited on the
upper and lower surfaces. (Figure 4A)
c) At this and subsequent stages, associated electrical circuits may be defined in
the wafer.
d) Layer 21 is selectively etched to define the beam and its pivots 4. (Figure 4B)
e) layers of suitable metals such as chromium typically 0.1 micrometer thick followed
by gold typically 0.3 micrometers thick, are deposited on the top insulating layer
and patterned photolithographically to define fixed electrodes 6 and beam plating
3 (Figure 4C).
f) Photoresist is spun onto the lower insulator and the insulator is patterned and
etched to define the open channel 7 (Figure 4D).
g) A layer of photoresist typically 3 micrometers thick is deposited to define the
gap between the upper and lower contacts, and is patterned to define the beam plating
area 3.
h) A plating base 23, typically 0.5 micrometers thick, and of any suitable metal such
as gold, is deposited over the whole upper surface.
i) Photoresist 24 typically 4 to 5 micrometers thick is deposited over the top surface
and patterned to define edges of the top contacts 11A and 11B.
j) Exposed metal is plated with a suitable metal such as gold 25 to a thickness of
typically 3 micrometers which strengthens the beam to remove stress effects and also
provides the top contacts 11A and 11B.
k) resist 24, exposed plating base 23, and resist 22 are all removed.
l) The channel 7 and the cavity 8 are formed by further anisotropic etching.
[0025] The invention is not limited by the illustrated embodiment; other structures and
methods of manufacture thereof, together with suitable circuits within the inventive
principles will be apparent to those with skills in the art.
1. A flow sensing device comprising a substrate of semiconductor material and a micromachined
beam, the device having first and second major surfaces, the first surface further
defining a pivot about which the beam is able to twist under the action of fluid flow.
2. A flow sensing device according to claim 1 wherein there is a channel below one
half of the beam, the channel extending throughout the substrate from the first major
surface to the second major surface, and wherein there is a cavity in the substrate
below the other half of the beam, such that during operation of the device fluid may
flow through the channel thus exerting a force on one half of the beam and fluid fills
the cavity thus creating a first differential force across the beam.
3. A flow sensing device according to claim 2 wherein a second force may be applied
to the beam to compensate for the effect of the first differential force, the second
force being indicative of the fluid flow being measured.
4. A flow sensing device according to any preceding claim wherein the beam is pivoted
substantially at its mid-point.
5. A flow sensing device according to claim 4 wherein the beam comprises an insulating
film having a metallic coating, the beam further having electrical contacts formed
on its free ends.
6. A flow sensing device according to claim 5 including a circuit arrangement to compensate
for twisting of the beam as a result of fluid flow.
7. A flow sensing device according to claim 6 wherein the circuit arrangement includes
a feedback control system.
8. A flow sensing device according to claim 7 wherein the circuit further includes
means to perform a square root function within the feedback system.
9. A method of manufacturing a flow sensing device according to claim 1 including
the steps of:
(a) applying an electrically insulative material to the major surface of a silicon
wafer,
(b) selectively etching the electrically insulative material to define the beam and
its pivots,
(c) applying at least one metallic layer over the electrically insulating layer,
(d) photolithographically patterning at least one metallic layer to define electrodes
on the beam and their associated contact points,
(e) etching a channel through the device extending from the first major surface to
the second major surface,
(f) depositing a layer of photoresist to define a gap between the electrodes and their
associated contact points,
(g) depositing a metallic plating base over the whole upper surface of the device,
(h) depositing photoresist over the whole upper surface of the device to define the
edges of the electrodes,
(i) coating any exposed metal with a suitable metal for strengthening the beam to
remove stress effects, and
(j) removing any exposed plating base and photoresist.